The present invention relates to an anti-lock hydraulic brake system comprising a pedal-actuator braking pressure generator, including a braking pressure modulator comprising plunger systems which are inserted into the prssure fluid conduits leading from the braking pressure generator to the wheel brakes, an auxiliary-pressure supply system and of electromagnetically actuatable multidirectional control valves for the control of the plunger systems. The system further comprises wheel sensors and electronic circuits which serve to generate electric signals by logical combining and signal processing for the purpose of control of the multidirectional control valves as a function of the wheel rotational behavior and additional criteria.
Such a brake system is known from German published patent application No. 29 08 482. In this case, the braking pressure modulator also comprises in each controlled brake circuit a plunger system which is inserted into the pressure fluid conduit leading from the pedal-actuated braking pressure generator to the wheel brake. The auxiliary-pressure supply system required for the actuation of the plunger pistons comprises a hydraulic pump and a pressure fluid reservoir according to this German published application. The plunger system is composed of a control piston and a reaction piston. Prior to the commencement of control, the control piston is maintained by the auxiliary pressure in its end position in which the valve ball of a non-return valve at the head of the control piston is likewise maintained in its opened position by a tappet secured to the housing, and thereby releases the pressure fluid conduit from the braking pressure generator to the wheel brake. Upon the occurrence of a signal indicative of an imminent locked condition, the pressure chamber on the one side of the control piston will be connected by way of an electrically actuatable valve with the return line wherein displacment of the control piston and closure of the non-return valve results. By the action of the reaction piston and by deviation of the auxiliary-pressure medium, a chamber which is disposed at the frontal end of the control piston and is closed in this phase by the non-return valve will be enlarged, giving rise to a pressure decrease in the wheel brake.
This known plunger system is relatively complicated in that it requires at least two additional solenoid valves per control circuit and, moreover, a pressure accumulator because the auxiliary pressure must be available at all times. On failure of the auxiliary pressure source, normal braking continues to be possible, however, with an increased pressure fluid volume and a larger pedal travel. Similar circumstances apply for other known plunger systems inasmuch as they either require relatively complicated valve assemblies and/or complicated control elements and control valves.
It is the general object of the present invention to develop an anti-lock system which can be manufactured at minimum cost. Special importance is attached to the unrestricted operability of the brake system on failure or impairment of the energy supply or in the event of any other disturbance appearing prior to or during braking with slip control, while only the failure of the control, but not an impairment of the brake characteristics, is tolerated.